In the manufacture of so called sol-gel aluminum oxide ceramic articles and in particular aluminous abrasive grain, large volumes of water have been used to form the gels. These large volumes have been made necessary basically for 3 (three) reasons:
1. The prior art teaches that gels, particularly for abrasive purposes, made from gelable alumina powders should contain high levels (3-7 weight percent) of MgO. The MgO is introduced into the sol-gel as a solution of a magnesium salt. Such introduction of salt solutions dramatically increases the viscosity of the system. Therefore, to ensure good homogeneous mixing of the magnesium salt with the alumina gel it is necessary to work with a relatively dilute sol-gel so that upon addition of the magnesium salt the viscosity remains low enough so that efficient and good mixing remain possible. For conventional abrasive manufacture containing around 5% MgO, a solids content in the 15-25 weight % range may be conveniently worked with but becomes very tedious beyond that. See, for example, U.S. Pat. No. 4,314,827 and published British application 2,099,012A. PA1 2. The manufacture of gels from present commercially derived alumina monohydrate powders requires that some acid (e.g., nitric or formic) be added to the powder to allow the colloidal particles to disperse. In order to ensure that the acid can interact with all of the powder, the acid is diluted with a large volume of water and conventionally mixed in a high shear blender or other convenient mixer. If insufficient water is used, the gel becomes too viscous to homogeneously mix in the magnesia (as above) and also it is extremely difficult to gel agglomerates of unpeptized alumina powder. With conventional mixing it becomes very very tedious to ensure a homogeneous peptization beyond about 45 weight % solids content. Solids content referes only to alumina monohydrate content in the sol gel and neglects the magnesia salt. PA1 3. Relatively large volumes of solvent (almost always water but could be a mixture of methanl-water or many other possibilities) are required in conventional gel preparation in order to ensure proper formation of the gel so that a coherent glassy dried green form is obtained. If insufficient water (solvent) is present the resulting green-ware is not coherent and tends to crumble easily, not at all exhibiting the glassy like features of the high water containing gel. When these poorer quality green bodies are sintered, the product tends to have weak zones and "pock marked" surfaces. This becomes especially noticeable beyond about 50 weight % solids. PA1 1. energy cost to remove the water is extremely high because conventional dewatering such as filtration is difficult if not impossible once the gel is formed, PA1 2. large capital investments are needed to provide large scale dewatering equipment, PA1 3. volumes handled are large, and PA1 4. control of shrinkage and cracking during drying is extremely difficult.
The presence of large volumes of water has several major disadvantages:
In view of the drawbacks of these conventional low solids content gels, I was led to investigate means whereby high quality ceramic articles, particularly abrasive grain could be made at high solids (greater than 30%) levels. Thus an object of the invention is to produce a high solids content aluminum oxide based gel system which will as a result have much improved economics of drying (dewatering) and permit much more control of greenware shrinkage and cracking during drying.